Among various types of the rolling devices, when a rolling bearing is used under such a condition in which water mixes into a lubricating agent or the like (Non Patent Documents 1-5), in which slipping is involved (Non Patent Document 6), and in which power distribution occurs (Non Patent Document 7), premature separation may often be caused due to hydrogen embrittlement by decomposition of water or the lubricating agent to produce hydrogen, and entering of the produced hydrogen into steel. As hydrogen significantly reduces a fatigue strength of steel (Non Patent Document 8), even under a condition of elastohydrodynamic lubrication in which contact elements are separated by an oil film, cracks may be produced within and near a rolling surface where an alternating shear stress is maximized and propagated, resulting in premature separation. In order to respond to demands for downsizing, energy saving, and elimination of manpower in the future, use conditions of a rolling bearing tend to become increasingly severe, and it is expected that a higher resistance to hydrogen embrittlement is required. In a case of the rolling device such as a gear having greater slipping than a rolling bearing, an occurrence of similar premature separation is quite expected.
In recent years, with increasing demands for downsizing, weight saving, and an improvement of quietness, efforts for downsizing, weight saving, and an improvement of sealing properties within an engine compartment for electric components and auxiliary components of automobiles are also demanded. On the other hand, higher output and higher efficiency are more and more demanded as capabilities of the device itself. For electric components and auxiliary components within an engine compartment, a method of compensating output reduction resulted from downsizing by a high-speed revolution is employed. In the following, a rolling bearing for a fan coupling device, a rolling bearing for an automobile alternator, and a rolling bearing for an idler pulley will be described as examples of the rolling bearing for an automobile electric component or an auxiliary component.
A typical fan coupling device for an automobile has a housing in which a viscous fluid is enclosed and to an outer peripheral surface of which a blower fan is attached, being coupled to a rotor that is directly coupled to an engine via a bearing, and performs air blowing optimal to the engine temperature by controlling an amount of driving torque transmission from the engine and a revolution speed of the fan by utilizing a shear resistance of the viscous fluid that increases and decreases depending on ambient temperature. Accordingly, a rolling bearing for a fan coupling device requires capability of enduring variation in revolution in which the revolution speed changes from 1000 rpm up to 10000 rpm according to variation in engine temperature, as well as a highly inhospitable environment in which high-speed revolution is performed at a revolution speed of 10000 rpm or over under high temperature often at 180 deg C. or above in high-speed driving during summertime.
A typical automobile alternator has a function of generating power by receiving revolution of an engine with a belt, supplying power to an electric load of a vehicle, and charging a battery. Further, a typical idler pulley for automobiles is used as a belt tensioner of a driving belt that transmits the revolution of the engine to an auxiliary component of an automobile, and a function as a pulley giving a tensile force to a belt as a tensioner when an inter-shaft distance is fixed, and has a function as an idler used for changing the traveling direction of the belt and avoiding obstacles to reduce a volume of an engine compartment. The automobile alternator and the idler pulley for automobiles also require capability of enduring a highly inhospitable environment in which high-speed revolution is performed at a revolution speed of 10000 rpm or over under high temperature at 180 deg C. or above.
For lubrication of the rolling bearings for automobile electric components and auxiliary components, grease is mainly used. However, rapid acceleration/deceleration, and inhospitable use conditions such as high temperature and high-speed revolution cause a problem of premature separation of a rolling surface of the rolling bearing due to hydrogen embrittlement.
Further, a step-up gear or a reduction gear (also integrally referred to as “step-up/reduction gear”) in which a rolling bearing is employed is used in various devices. Examples of such gears include an reduction gear for industrial machine (circulating oiling), a reduction gear for robot (oil bath lubrication), a reduction gear for mill (circulating oiling or oil bath lubrication), a reduction gear for construction machine (oil bath lubrication), and a step-up gear for wind generator (circulating oiling or oil bath lubrication). Within the brackets, types of oil lubrication for gear units including the rolling bearing are shown, which specifically include oil bath lubrication, jet oiling, circulating oiling, oil-mist lubrication, air-oil lubrication, splash oiling, and hydraulically-actuated oil dipping, but are roughly categorized into oil bath lubrication and circulating oiling. When the rolling bearing is used under a condition in which water mixes into a lubricant, or a condition in which the rolling bearing involves slipping, water and lubricant are decomposed to produce hydrogen. By the produced hydrogen entering into steel, premature separation may often be caused due to hydrogen embrittlement. When metallic contact occurs between contact surfaces of the contact elements and a newly-formed metallic surface is exposed, production of hydrogen due to decomposition of water and the lubricant and entrance of the hydrogen into steel is promoted.
Further, a transmission for automobiles is a device for converting power of an engine into optimal torque and revolution speed and transmits it to vehicle wheels, and a rolling bearing used in such a transmission is desired to follow a large load capacity and high rotation capacity, in particular a radical change in the revolution speed that accompanies shift transmission. Moreover, as the bearing bites a foreign matter within a transmission casing, an improvement of durability is sought by preventing a foreign matter from entering by attaching a tight seal or providing a special thermal treatment. Even though a tight seal is attached, a main purpose of the tight seal is to prevent a foreign matter from entering. The tight seal does not have a function of preventing entrance of a fluid, and therefore oil filled in the transmission comes into the bearing through a space around the tight seal. Until the oil comes into the bearing, grease previously enclosed in the bearing provides lubrication.
In recent years, as in a case of a CVT (continuously variable transmission), under an environment in which a lubricant with low viscosity is used as transmission oil, there are cases in which separation occurs in bearing races far earlier than separation due to fatigue and biting of foreign matters occurs (premature separation described above). This type of separation is characterized by a number of cracks microstructurally produced in a surface where the separation occurs. Such cracks are not observed in conventional separation. As described above, it is considered that this type of separation occurs because slipping between a race and a rolling body occurs, hydrogen is produced by decomposition of components of grease within a bearing or of transmission oil that has come into the bearing, and the produced hydrogen enters steel to cause embrittlement of the steel. The lower the viscosity of lubricant components is, the more easily slipping occurs. Therefore, decomposition of the components and premature separation due to the hydrogen embrittlement occur more easily.
Further, due to its intended use, a hub bearing for an automobile is used not only for driving in fine weather, but under extremely severe use environments such as driving in rainy weather, or along a rough road and coast. Entrance of moisture and a foreign matter into the hub bearing is restrained by sealing, but not completely. Therefore, it is not possible to completely prevent moisture and a foreign matter from entering the hub bearing and from being mixed into a lubricating agent such as grease enclosed in the bearing. Moreover, reduction of torque of the hub bearing is demanded in view of energy saving, and light-contact sealing is considered as one method. This increases possibility of entrance of water.
In the hub bearing for an automobile, when used under such a condition in which water is mixed into the lubricating agent, an inhospitable use condition such as rapid acceleration/deceleration, and a condition in which slipping is involved, premature separation due to the hydrogen embrittlement occurs in a rolling surface of the hub bearing, and it becomes difficult to use the hub bearing for a long period of time.
Use of bearing steel, or the like, with an increased Cr content is proposed as one conventional technique for improving a resistance of the rolling bearing to the hydrogen embrittlement to prevent occurrence of the premature separation described above (see Patent Document 1). According to this technique, Cr whose content in steel is increased is combined with oxygen over the rolling surface, an oxide layer (passive film) of Cr is formed over the rolling surface, and this oxide layer prevents hydrogen from entering steel, and thus prevents premature separation due to hydrogen embrittlement from occurring. Further, a method of adding a passivation agent (see Patent Document 2), or bismuth dithiocarbamate (see Patent Document 3) to grease is proposed as an example of a method for preventing the premature separation by modification of the lubricating agent.